The invention relates to a wind turbine, a wind turbine controller, and a method of controlling a wind turbine. It relates in particular to a wind turbine suitable for use in large scale electricity generation on a wind farm, for example.
In FIG. 1, the solid line 10 of the graph 12 illustrates the variation of power output with wind speed (measured at the height of the hub) for a typical wind turbine used for large scale electricity generation. As is well known in the art, for a wind turbine with a doubly fed induction generator (DFIG), at very low wind speeds, typically between 0 and 3 or 4 m/s, the wind turbine idles. That is to say, the blades of the wind turbine do not rotate such that the wind turbine generates electrical power. This is because there is not considered to be enough energy available from the wind to generate power from the wind turbine. This is the low wind idling region 14. At a lower cut-in wind speed Vmin, typically between 3 or 4 m/s, the blades of the wind turbine start to rotate to generate power at part or partial (electrical) load. This is called the part load region 16. The part load region is typically between wind speeds of 3 or 4 m/s and 12 or 13 m/s. For a wind turbine with a full converter, there may not be an idling region where the blades rotate but no electrical power is generated from the wind turbine. In a typical wind turbine having a full converter, as soon as the force of the wind overcomes the frictional forces in the drive train and the rotor blades start rotating, the wind turbine will start generating electrical power. Thus, in the present invention, the lower cut-in wind speed Vmin on a wind turbine having a full converter may be defined as the wind speed at which the blades start to rotate and electrical power is generated.
As the wind speed increases, the wind turbine enters the full load region 18, at and above the rated wind speed Vr, where the blades of the wind turbine rotate to produce substantially the same power at any wind speed in this region. That is to say, in the full load region, the wind turbine generates the maximum permissible power output of the generator and the power output is substantially independent of the wind speed. The power output is regulated to be substantially constant. The full load region is typically between wind speeds of 12 or 13 m/s and 25 m/s. Finally, at high wind speeds at or above Vmax, the upper cut-out wind speed, the wind turbine idles (the blades of the wind turbine do not rotate to generate electrical power; and the generator of the wind turbine is disconnected from the electricity distribution network or grid) and this is called the high wind idling region 20. The upper cut-out wind speed, Vmax, is typically 20 m/s or 25 m/s. At these high wind speeds, the wind turbine is shut down for safety reasons, in particular to reduce the loads acting on the wind turbine, which can damage it.
Wind turbines usually have mechanisms for changing the aerodynamic effect of the wind acting on their blades. These mechanisms include blade pitching (where each blade of a wind turbine is rotated about its longitudinal axis) or providing moveable flaps as part of the wind turbine blade. These mechanisms are used in particular ways at particular wind speeds.
Commonly, blade pitching is used to compensate for variations in wind speed over the height of the wind turbine caused by so-called wind shear. Typically, to compensate for this, wind turbine arrangements include blades that pitch in a cyclical fashion as the blades rotate at rated wind speeds, such as in US patent application No. US 2008/0206055. This variation in wind speed over the height of the wind turbine also results in loads acting on wind turbine blades varying across the blades and blade pitching is known to reduce the resultant asymmetric loading across a wind turbine in these circumstances such as described in European patent application No. EP 1978246, US patent application No. US 2007/0286728, US patent application No. US 2007/0212209, US patent application No. US 2006/0145483, US patent application No. US2002/004725, and Bossanyi, E. A. “Individual Blade Pitch Control for Load Reduction”; Wind Energy, Volume 6, pages 119-128.
In other arrangements the same pitch angle is applied to all of the blades, such as described in European patent application No. EP 1666723. In this system, a common pitch-angle is applied to all of the blades with the aim of reducing stresses on the blades at low or full loads.
Blade pitching is also used to reduce forces in wind turbine blades at high winds such as in European patent application No. EP 1890034 in which there is interdependence between the pitch angles of the blades under these wind conditions; and in German patent application No. DE 102005034899 where the blades of a wind turbine are all pitched together to shutdown the wind turbine. The wind turbine described in European patent application No. EP 1630415 includes another mechanism for reducing forces during severe wind conditions, such as a heavy storm or hurricane. The wind turbine in this document has outboard blade sections which are folded in to reduce the lift forces under these extreme circumstances.
One arrangement describing the use of flaps in wind turbine blades to alter the aerodynamic properties of the blade is described in US patent application No. 2007/0003403. The aim of the described arrangement is to allow the turbine to operate at wind speeds above the upper cut-out wind speed at which the turbine would have otherwise been stopped to prevent excessive load being applied to the wind turbine. The flaps of particular blades in a particular rotational position are adjusted so that they adopt the position of the flaps of other blades when they were in the same rotational position. In other words, there is interdependence between the flap positions.
It would be advantageous if a wind turbine had reduced mechanical loads at high wind speeds above the upper cut-out wind speed. This would help to prevent damage to the wind turbine. Furthermore, the wind turbine could be built to resist lower extreme loads and the cost to build the wind turbine would be reduced.
The inventor of the system described herein is the first to appreciate that blades of a wind turbine may be independently controlled of the other blades (such as by pitching the blades) and/or by independently controlling one or more components of each blade (such as by moving flaps or tabs of each blade) in order to reduce mechanical loads of one or more components of the wind turbine (such as the blades or tower) when wind speed acting on the blades is above cut-out.